This invention relates to navigational systems. More particularly, this invention relates to navigational systems comprising a series of satellites positioned in geosynchronous orbit above the earth which transmits a radio carrier modulated by a tone to be received by mobile craft to obtain a geographical fix.
Presently there exist many different types of navigational systems which provide a means for a mobile craft to obtain a geographical fix on the surface of the earth. U.S. Pat. No. 3,384,891 (Anderson) discloses a navigational system having a ground station which transmits ranging signals to a satellite which, in turn, retransmits such signals to the mobile craft and the ground station. The mobile craft repeats the ranging signals received from the satellite and retransmits them to the satellite which, in turn, retransmits them to the ground station. The ground station receives both the transmissions of the repeated ranging signals from the satellite and measures the time interval therebetween. From this time interval, the ground station computes the range from a known position of the satellite to the mobile craft. The same interrogation message is transmitted from the ground station to a second satellite at a different location to obtain two computed ranges from the mobile craft to two known positions of the satellites. The two range measurements using two different satellites provides two circles whose intersection is the position of the mobile craft.
The method of repeating ranging signals to obtain a geographical fix requires an interrogation signal to be transmitted by the mobile craft. Such interrogation reveals the position of the mobile craft to others, and thus is undesirable. The ranging method also requires a transmitter to be placed on the mobile craft. The requirement of such a transmitter increases the cost and reduces the reliability of the navigational system and is therefore also undesirable.
A further disadvantage of the ranging method is the possibility of an overload condition due to each satellite receiving interrogation signals on the same frequency from numerous mobile craft. Such an overload condition reduces the accuracy of the system and could possibly cause some mobile craft to lose complete navigational reference.
To obviate the need for an interrogation signal, subsequent prior art navigational systems utilized synchronized transmitters placed at known positions around the world. The transmitters transmit frequencies having the same phase. The mobile craft receives such frequencies and compares the phase difference therebetween to obtain a geographical fix. For example, a position locating system known as OMEGA, has been developed by the U.S. Navy and is described in detail in a publication entitled "OMEGA, a World-Wide Navigational System", prepared by the OMEGA Implementation Committee and published by Pickard & Burns Electronics, 103 4th Avenue, Waltham, Mass. In the OMEGA System, a plurality of transmitters located in terrestrial stations at known points around the surface of the earth transmit a sequential signal that switches between three primary frequencies. The several transmitters are synchronized so that at any particular instance of time, only one transmitter emits a continuous wave signal at a particular frequency. During succeeding intervals of time, the transmitter frequencies are shifted whereby the frequency transmitted from the first station during the first interval is transmitted from succeeding stations during succeeding intervals. Each terrestrial transmitter contains oscillators which are phase-locked to a standard time, common to all terrestrial stations, to enable all terrestrial transmitters to transmit with a common phase.
Conventional OMEGA receiving equipment designed for use by mobile craft generally contains stable oscillators which are individually phase-locked to the separate OMEGA signals received from three different terrestrial stations. The equipment determines the position of the mobile craft by measuring the phase difference between the pairs of these phase-locked oscillators. Each measured phase difference corresponds to a difference in the distance between two terrestrial transmitters, and thus locates the mobile craft on a hyperbolic line of position on an OMEGA navigation chart, as is well known in the art. Phase measurements made between each different pairs of oscillators locate the mobile craft on the different hyperbolic lines of position, which intersect to identify the position of the mobile craft on the chart.
Other navigational systems use techniques for determining the position of the mobile craft in a manner similar to the OMEGA System. For example, U.S. Pat. No. 3,789,409 (Easton) describes a navigational system comprising a series of satellites which transmit multi-frequency signals derived from the same oscillator. The stable oscillator is phase synchronized with the receiver equipment on the mobile craft. The equipment computes the phase differences between the signals received from the satellites and the signals produced within the receiver to determine the distance between the mobile craft and the satellites. Hyperbolic lines of positions can then be plotted, and the position of the mobile craft determined.
A major problem associated with the above systems is the necessity of synchronizing the phase of the receiver equipment signals to the phase of the transmitter equipment. Atomic clocks are being utilized to help overcome such a problem. More particularly, the Easton patent and U.S. Pat. No. 3,643,259 (Enter) incorporate an atomic clock to provide such synchronization. It is readily apparent that a momentary failure of either the atomic clock in the receiver equipment on the mobile craft or in the transmitter equipment will render the system inoperable. Furthermore, the phase drift inherent in atomic oscillators would also render the system inaccurate. Thus, such a volatile and unstable system is most undesirable.
Therefore it is an object of this invention to provide an apparatus and method which overcomes the aforementioned inadequacies of the prior art devices and methods and provides an improvement which is a significant contribution to the advancement of the navigational system art.
Another object of this invention is to provide a navigational system wherein the transmitter equipment does not interrogate the receiver equipment on the mobile craft and thus does not reveal the position of the mobile craft.
Another object of this invention is to provide a navigational system which eliminates the overload conditions associated with ranging methods utilizing interrogation signals.
Another object of this invention is to provide a navigational system which obviates the need for atomic clocks to provide synchronization of the transmitter equipment and the receiver equipment on the mobile craft.
Another object of this invention is to provide a navigational system which is not volatile to loss of power or loss of synchronization.
Another object of this invention is to provide a navigational system for long distance navigation of mobile craft around the world.
Another object of this invention is to provide a navigational system for both surface mobile craft and airborne mobile craft.
Another object of this invention is to provide a navigational system which overcomes the altitude error associated with airborne mobile craft.
The foregoing has outlined some of the more prominent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications to the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description describing the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.